Fact:
Common sense seems to indicate that a respirator filter acts like a screen.
If one puts a bunch of dirt on a screen and shakes it out, the result is
that the bigger chucks/particles of dirt are caught on top of the screen
while the smaller ones fall through. Though this may be the case for larger
particles, this screen theory doesn't hold true for the smaller particles
when it comes to a filtering facepiece respirator. When smaller particles
enter the filter area, they end up moving in a random motion and bouncing
about - causing the particles to get caught in the filter medium.

Impaction refers to the process when the larger particles actually collide,
or make impact, with the filter media and become trapped.

Diffusion refers to the process when the particles break apart and move in a
random pattern, becoming caught in the filter media. So the smaller
particles don't break through as easily as one might think that they would.